Cd1−xZnxTe Research Articles

Assessment of deep levels with selenium concentration in Cd1–xZnxTe1–ySey room temperature detector materials

Incorporation of Se into Cd1−xZnxTe (CZT) to form the quaternary compound semiconductor Cd1−xZnxTe1–ySey (CZTS) has proven to be an effective solution for compensating the major flaws associated with CZT, including poor homogeneity and high concentrations of electronically active deep levels that limit the performance of CZT detectors. In order to investigate how deep levels are affected by the Se concentration in CZTS, we performed photoinduced current transient spectroscopy (PICTS) measurements on CZTS crystals grown by the traveling heater method (THM) with 10% atomic Zn and varying atomic percentage of Se from 1.5% to 7.0%. The PICTS scans for up to 4% Se showed an exponential reduction in the capture cross section of deep levels associated with Te secondary phases in conjunction with an increase in a deep level positioned near the mid-gap, which initially increases the electron trapping time before degrading again at higher Se concentrations. The PICTS peaks present in 7% Se were anomalous relative to the other crystals and are expected to originate from transition metal impurities found in the lower-purity CdSe precursor material.

Extreme γ-Ray Radiation Tolerance of Spectrometer-Grade CsPbBr3 Perovskite Detectors.

The perovskite compound CsPbBr3 has recently been discovered as a promising room-temperature semiconductor radiation detector, offering an inexpensive and easy-to-manufacture alternative to the current benchmark material Cd1-x Znx Te (CZT). The performance of CsPbBr3 sensors is evaluated under harsh conditions, such as high radiation doses often found in industrial settings and extreme radiation in space. Results show minimal degradation in detector performance after exposure to 1Mrad of Co-60 gamma radiation, with no significant change to energy resolution or hole mobility and lifetime. Additionally, many of the devices are still functional after being exposed to a 10Mrad dose over 3 days, and those that do not survive can still be refabricated into working detectors. These results suggest that the failure mode in these devices is likely related to the interface between the electrode and material and their reaction, or the electrode itself and not the material itself. Overall, the study suggests that CsPbBr3 has high potential as a reliable and efficient radiation detector in various applications, including those involving extreme fluxes and energies of gamma-ray radiation.

Toward Development of CdZnTe and CdMnTe Thin Film‐Based Solar Cells: Optimized Properties and a Comparative Performance Exploration

AbstractThin‐film solar cells (TFSCs) are garnering attention for their unstoppable growth as a sustainable energy source as well as their innovations and indispensability. TFSCs' efficiency has plateaued, necessitating the development of novel, practical, and inexpensive alternative energy sources. CdTe material lacks the band gap for the top layer. It can still be used as an absorber top layer in tandem photovoltaic cells by forming two new, promising materials, Cd1–xZnxTe (CZT) and Cd1–xMnxTe (CMT). Consequently, CZT and CMT based PV cells have been explored, and an attempt has been made to provide an alternative to cadmuim telluride. After exploring the physical properties of CZT and CMT thin films, CZT and CMT based photovoltaic cells were fabricated using the vacuum evaporation method. Both devices (with low absorber layer thickness, 1.05 µm) showed a power conversion efficiency of 6.67% and 6.11%, respectively. The depletion layer width and dopant densities of both photovoltaic cells were also determined by capacitance–voltage measurements. The quantum efficiencies were measured to identify the specific losses that are causing the measured short circuit current density to deviate from its ideal value. The reproducibility of the devices was also tested, and it is found to be good with an 8–12% deviation.

Defects Characterization of HgCdTe and CdZnTe Compounds by Positron Annihilation Spectroscopy

Infrared cooled photodetectors must operate at higher temperatures to reduce their size, weight and power consumption (SWaP context). Their stability and image quality are then challenged by extra electrical activity of crystal defects. Knowledge of defect populations is mandatory to improve the material quality of the epitaxial Hg1−xCdxTe (MCT) active layer and the Cd1−xZnxTe (CZT) substrate. Positron annihilation spectroscopy with a slow positron beam was used to study near-surface open-volume defects profiles. Low- and high-momentum fractions (\(S\),\( W\)) were used to characterize the Doppler broadening of the 511 keV electron-positron pair annihilation-line as a function of the positron implantation energy \(E\). The results show that three regions can be identified beneath the surface of the as-grown non-optimized MCT layer. The quasi-linear relationship between the annihilation characteristics in the regions suggests that the defect populations mainly correspond to the same open-volume defect in different concentrations. The probed defect is thought to be related to the mercury vacancy. This hypothesis is discussed in an original way with near-surface elemental profiles using scanning transmission electron spectroscopy combined to energy dispersive x-ray spectroscopy (STEM-EDX). Afterwards, this approach is extended to CZT substrates showing that surface and bulk properties of those fabricated by LYNRED tend to match those that are state-of-the-art. A common open-volume defect is probed, in concentration estimated by Hall effect around 1015 cm−3 and thought to be related to the cadmium vacancy.

Vertical Gradient Freeze Growth of two inches Cd1−xZnxTe1−ySey ingots with different Se content

Two inches in diameter and four inches in length quaternary alloy Cd1−xZnxTe1−ySey (CZTS) ingots have been grown by the Vertical Gradient Freeze Method (VGF) for two different Se concentrations (y = 0.02 and 0.07). The compositional and crystallographic properties, as well as defect morphology of the ingots have been analysed using EDX, TXRF, XRD and IR microscopy. Despite of using these two extreme Se values proposed in the literature, CZTS continues to maintain the proper zinc blende structure of CZT. However, the addition of selenium has the effect of shifting the diffraction peaks to higher angles, or what is the same, to reduce the Cd1−xZnxTe (CZT) constant lattice. It is observed that the addition of Se also favours the reduction of bubble formation along the ingots due to cadmium deficiency and change in Zn concentration along the growth CZTS ingot direction respect to CZT ingot. The VGF growth method provides great confidence and reproducibility since both ingots share similar features in terms of dimensions and defects formation.

Positron lifetime spectroscopy of defect structures in Cd1–x Zn x Te mixed crystals grown by vertical Bridgman–Stockbarger method

Positron annihilation lifetime spectroscopy was used to examine grown-in defects in Cd1–x Zn x Te mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high-pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X-ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open-volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe-based materials is discussed.

Spectroscopic and electrical analysis of vacuum co-evaporated CdxZn1-xTe thin films

Abstract CdxZn1-xTe (CZT) films with different compositions were grown by thermal co-evaporation method, in which cadmium telluride (CdTe) and zinc telluride (ZnTe) were used as the source materials. The structural, morphological and optical properties of the films were characterized by x-ray diffraction (XRD), UV–Visible spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL) and Raman spectroscopy. Detailed analysis has shown that films were having cubic phase with orientation along (111) plane and the optical band gap reduced from 2.17 to 1.53 eV when x was varied from 0 to 1. SEM micrographs confirmed the homogeneity of the films. Depending on the composition, Raman peaks of CZT samples were mixture of the fundamental modes of the CdTe and ZnTe and indicate “trigonal” lattice of Tellurium (Te). Analysis of PL spectra suggested tellurium as isoelectronic exciton traps, recombination of carriers in the surface traps and donor-acceptor pair present in the samples. Under electrical properties, p to n-type conversion was witnessed for x = 0.4 and above. Optical bad gap of about 1.60 eV with minimum Urbach energy (28.88 meV) and favourable electrical properties has led to conclude that CZT films with x = 0.8 may be suitable for solar cell absorber.

Internal Atomic-Scale Structure Determination and Band Alignment of II–VI Quantum Dot Heterostructures

This work shows that ZnTe/CdSe core/shell quantum dots synthesized by a standard literature procedure in actuality have an alloyed CdxZn1–xTe core. We employ X-ray absorption spectroscopy (XAS) at ...

Characterization of Cd1-xZnx Te (0≤x≤1) Nanolayers Grown by Atomic Layer Deposition on GaSb and GaAs (001) Oriented Substrates

ZnTe, CdTe, and the ternary alloy CdZnTe are important semiconductor materials used widely for the detection of an important range of electromagnetic radiation as gamma ray and X-ray. Although, recently these materials have acquired renewed importance due to the new explored nanolayer properties of modern devices. In addition, as shown in this work they can be grown using uncomplicated synthesis techniques based on the deposition in vapour phase of the elemental precursors. This work presents the results obtained from the deposition of nanolayers of these materials using the precursor vapour on GaAs and GaSb (001) substrates. This growth technique, extensively known as atomic layer deposition (ALD), allows the layers growth with nanometric dimension. The main results presented in this work are the used growth parameters and the results of the structural characterization of the layers by the means of Raman spectroscopy measurements. Raman scattering shows the peak corresponding to longitudinal optical (LO)-ZnTe, which is weak and slightly redshift in comparison with that reported for the ZnTe bulk at 210 cm–1. For the case of the CdTe nanolayer, Raman spectra presented the LO-CdTe peak, which is indicative of the successful growth of the layer. Its weak and slightly redshift in comparison with that reported for the CdTe bulk can be related with the nanometric characteristic of this layer. The performed high-resolution X-ray diffraction (HR-XRD) measurement allows to study some important characteristics such as the crystallinity of the grown layer. In addition, the HR-XRD measurement suggests that the crystalline quality has dependence on the growth temperature.

Electrical, charge transients and photo response study of as-deposited and phosphorus implanted Cd1-xZnxTe devices for PV applications

High quality polycrystalline Cd1-xZnxTe (CZT) thin films with compositions (x = 0, 0.25, 0.5, 0.75, 1) were deposited on glass substrates by thermal evaporation technique followed by phosphorus implantation schedules at 800 keV and 1200 keV, carried out on the same samples. Detailed electrical properties of as deposited and implanted CZT films were investigated as a function of Zn content, such as, current-voltage (I–V), Charge based Deep Level Transient Spectroscopy (Q-DLTS), trap level energies (ET), Transient of Photo Voltage (TPV) and kinetics of photo current with variable light intensity ranges performed to study and evaluate the potential of this scheme for potential usage in the development of solar cells. It was observed that phosphorus implant increases the electrical conductivity, consequently, provides sufficiently higher carrier concentration for Photo Voltaic (PV) applications. The resistivity, carrier dynamics, charge transients, deep/trap assisted levels and intensity (visible light) controlled photo-responses, of CZT films are found to be tailored by selecting suitable Zn content and specific energy of phosphorus dopant, which may have ramifications for process development in CZT-based PV technology.

Revealing Te inclusions on VGF-grown CdZnTe crystals by using low energy argon ion irradiation: Influence of the substrate temperature

Cd1−xZnxTe (CZT) crystals find interesting uses in a variety of detector-type applications. However, it is well known that the concentration and distribution of Te inclusions within a device are one of the major contributions to the degradation of CZT detectors. In addition, Te precipitates on the surface can also increase the leakage current, thereby deteriorating the device performance.Our previous studies clearly revealed a pronounced effect when CZT surfaces were irradiated with Ar ions at room temperature. Even for small fluences, a removal of polishing scratches on the sample surfaces was observed. Correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. Te inclusions also tend to aggregate after the irradiation process and are also eliminated from the surfaces for the highest ion fluences.In this work we continue our previous work by studying the effect of the temperature of the sample in the range 300–600 K during Ion Beam Sputtering of CZT crystals surfaces at different fluences, by means of Scanning Electron Microscopy, Atomic Force Microscopy, and μ-Photoluminescence. It is expected that increasing the temperature of CZT samples during the irradiation process will enhance the beneficial effects this technique has shown when trying to reduce surface Te inclusions and to increase the luminescence properties of CZT crystal surfaces.

Study of material uniformity in high-resistivity Cd[formula omitted]Zn[formula omitted]Te and Cd[formula omitted]Mn[formula omitted]Te crystals

In the last decade Cd1−xMnxTe (CMT) has been considered as a very promising material for room-temperature radiation detectors. Manganese has a better segregation compared to that of zinc in Cd1−xZnxTe (CZT), in addition MnTe has a wider bandgap than ZnTe enabling broader band “tunability” in the ternary alloy. In this work, we provided a general comparison of these materials grown in similar conditions, focusing, on material composition uniformity and electric properties under dark conditions. Results of material uniformity analyses clearly confirm better homogeneity of CMT crystals, both laterally and in-depth. In addition to material studies, the current–voltage (I–V) characteristics of both In/CZT/In and In/CMT/In metal–semiconductor–metal (MSM) structures were characterized. The results revealed significantly lower variations in contact-to-contact currents in CMT compared to that in CZT (10% and 25% variance, respectively). The temperature behavior of I–V in In/CZT/In and In/CMT/In devices was investigated with calculation of dark current activation energy. The activation energies of dark current mechanisms in CMT were found to be similar at both polarities, whereas there was a 0.2 eV polarity-related difference in CZT based structures.

Impact of surface treatments on I–V characteristics in Cd1−xZnxTe and Cd1−xMnxTe crystals

In this work, we focused on studying the impact of different surface treatments for Cd1−xZnxTe (CZT) and Cd1−xMnxTe (CMT) samples with indium evaporated contacts, specifically: mechanical polishing (MP), mechano-chemical polishing (MCP) and mechanical polishing followed by the chemical etching (MP+Et). We examined 3 samples in each group of Cd0.85Zn0.15Te and 3 samples in each group of Cd0.93Mn0.07Te. Devices were consecutively characterized by current–voltage dependence (I–V). The measured characteristics revealed that for CMT both surface treatments, including chemical exposure to bromine ethylene glycol in MCP or bromine methanol in etching, resulted in linear I–V characteristic consistent with the bulk resistance. At the same time, chemical treatments of CZT samples dramatically increased their current densities (up to several μA/cm2).

Numerical Modelling of Front Contact Alignment for High Efficiency Cd1-xZnxTe and Cd1-xMgxTe Solar Cells for Tandem Devices

AbstractWide bandgap Cd1-xZnxTe (CZT) and Cd1-xMgxTe (CMT) have drawn attention as top cells in tandem devices. These materials allow tuning of the band gap over a wide range by controlling the Zn or Mg concentration with little alteration to the base CdTe properties. Historically, CdS has been used as a heterojunction partner for CZT or CMT devices. However, these devices show a significant lower open circuit voltage (VOC) than expected for wide bandgap absorbers. Recent modelling work suggests that poor band alignment between the CdS emitter and absorber results in a high concentration of holes at the interface, which increased recombination and limits the VOC. This recombination should be exacerbated for wider bandgap absorbers such as CZT and CMT. In this study, we use numerical simulations with SCAPS-1D software to investigate the band alignment in the front contacts for wider bandgap CdTe based absorbers. Results show that by replacing the CdS with a wide bandgap emitter layer, the VOC can be greatly improved, though under certain conditions, the fill factor remains sensitive to the location of the emitter conduction band. As a result, different transparent front contacts were also investigated to determine a device structure required to produce a high performance CZT or CMT top-cell for tandems devices.

Investigation on Relations of Twins and Inclusions Distribution with Growth Solid‐Liquid Interface in Cd0.96Zn0.04Te Crystal

AbstractTwins and inclusions remain major problems affecting the availability and cost of Cd1‐xZnxTe (CZT) substrates. In this paper, twins and inclusions distribution are investigated by visible light photograph, field emission scanning electron microscopy and infrared transmission microscopy. Moreover, the growth solid‐liquid interface of the CZT crystal is characterized by a zinc concentration distribution scanner designed by our laboratory. It is found that twins in the CZT crystals are much related with the unstable solid‐liquid interface, and the distributions of inclusions are also affected by the solid‐liquid interface shape. Based on these experimental results, some factors affecting the formation of twins and inclusions are discussed.

Effects of substrate temperature on morphological, structural and photo-electric properties of Cd1−xZnxTe polycrystalline films

Cd1−xZn x Te (CZT) films were deposited on ITO substrates at different substrate temperature by repeatedly RF magnetron sputtering with a Cd0.9Zn0.1Te target. The effect of substrate temperature on physical properties of CZT films were systematically investigated. The morphology of CZT films were characterized by atomic force and scanning electron microscopes. The grain size increased and grain boundaries decreased with substrates temperature increasing. XRD studies revealed that as-grown CZT films were polycrystalline with (111) preferential orientation and the crystallization quality was improved with the increase of substrate temperature. Optical analysis of CZT films showed the band gap decreased with the substrate temperature increasing. A1 and A2 detectors were fabricated based on the CZT films deposited at 300 and 400 °C, respectively. The electrical and X-ray response properties at room temperature of two detectors were investigated. The dark resistivity is 1.49 × 108 and 4.26 × 108 Ω cm for A1 and A2 detectors, respectively. The X-ray sensitivity of CZT films deposited at 400 °C was higher than that of CZT films deposited at 300 °C.

Low leakage current Ni/CdZnTe/In diodes for X/[formula omitted]-ray detectors

The electrical characteristics of the Ni/Cd1−xZnxTe/In structures with a metal–semiconductor rectifying contact are investigated. The diodes, fabricated on the base of In-doped n-type Cd1−xZnxTe (CZT) crystals with resistivity of ∼1010Ω⋅ cm, have low leakage current and can be used as X/γ-ray detectors. The rectifying contact was obtained by vacuum deposition of Ni on the semiconductor surface pretreated with argon plasma. The high barrier rectifying contact allowed us to increase applied reverse bias voltage up to 2500 V at the CZT crystal thickness of 1 mm. Dark (leakage) currents of the diodes with the rectifying contact area of 4 mm2 did not exceed 3–5 nA at bias voltage of 2000 V and room temperature. The charge transport mechanisms in the Ni/CZT/In structures have been interpreted as generation–recombination in the space charge region within the range of reverse bias of 5–100 V and as currents limited by space charge at both forward and reverse bias at V >100 V.

On determination of Cd1–xZnxTe composition from an analysis of the 4.2, 77 and 295 K edge photoluminescence spectra

On determination of Cd1–xZnxTe composition from an analysis of the 4.2, 77 and 295 K edge photoluminescence spectra

Cd 1-x Zn x Te photodetectors with transparent conductive ZnO contacts

High quality Cd1-xZnxTe (CZT) films were prepared using the close-spaced sublimation (CSS) technique. CZT film UV (ultraviolet) photodetectors were fabricated with B and Ga co-doped ZnO (BGZO) transparent conductive interdigitated contacts. The contact properties of BGZO/CZT were investigated by the transmission line model (TLM). The results indicate that a good ohmic contact is formed between BGZO and CZT with a very low contact resistivity of about 0.26 Ω·cm2. Compared with CZT photodetectors with Au contacts, the detectors with BGZO contacts show a higher value of UV photo response.

Raman Characterisation of Cd1-xZnx Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation

Raman Characterisation of Cd_1-xZn_x Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation